August 9, 2017
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for July 2017

1 Major Seismic Activities

* On July 1, there was a magnitude (M) 5.1 earthquake under the central and eastern Iburi region, Hokkaido. Maximum seismic intensity 5 Lower was observed in Hokkaido, and damage with injured persons and so on was caused.

* On July 2, there was an M4.5 earthquake in the Aso region, Kumamoto prefecture. Maximum seismic intensity 5 Lower was observed in Kumamoto prefecture.

* On July 11, there was an M5.3 earthquake in the Kagoshima Bay. Maximum seismic intensity 5 Upper was observed in Kagoshima prefecture, and damage with injured persons and so on was caused.

• Seismic Activities for All of Japan in July 2017 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On July 1, there was an M5.1 earthquake at a depth of approximately 25 km under the central and eastern Iburi region, Hokkaido. The focal mechanism had a compression axis in a WNW-ESE direction. This event occurred within the crust.

• Hokkaido Region
• Earthquake under the Central and Eastern Iburi Region on July 1

to supplementary reports

(2) Tohoku Region

* On July 7, there was an M4.9 earthquake at a depth of approximately 65 km, offshore Fukushima prefecture. The focal mechanism had a compression axis in an NW-SE direction. This event occurred within the Pacific plate.

* On July 20, there was an M5.8 earthquake at a depth of approximately 45 km, offshore Fukushima prefecture. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and the continental plates.

• Tohoku Region
• Earthquake Offshore Fukushima Prefecture on July 7
• Earthquake Offshore Fukushima Prefecture on July 20

to supplementary reports

(3) Kanto-Chubu Region

* In the GNSS data for the Tokai region, there has been no change of activity which is likely to be associated with the Tokai earthquake.

• Kanto - Chubu Region
• Secular variation of benchmark 2595 (Omaezaki City)
• Continuous GNSS Monitoring in the Area from Kakegawa to Omaezaki

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity.

• Kinki-Chugoku-Shikoku Region

to supplementary reports

(5) Kyushu-Okinawa Region

* On July 2, there was an M4.5 earthquake at a depth of approximately 10 km in the Aso region, Kumamoto prefecture. The focal mechanism showed a strike-slip fault type with a tension axis in an NW-SE direction. This event occurred within the crust.

* On July 11, there was an M5.3 earthquake at a depth of approximately 10 km in the Kagoshima Bay. The focal mechanism showed a strike-slip fault type with a tension axis in a WNW-ESE direction. This event occurred within the crust.

• Kyushu Region
• "The 2016 Kumamoto Earthquakes" (1)
• "The 2016 Kumamoto Earthquakes" (2)
• Earthquake in the Kagoshima Bay on July 11
• Okinawa Region

to supplementary reports

(6) Other Regions

* On July 13, there was an M6.3 earthquake at a depth of approximately 600 km under the western Japan Sea. The focal mechanism had a compression axis in the subducting direction of the Pacific plate. This event occurred within the Pacific plate.

• Earthquake under the Western Japan Sea on July 13

to supplementary reports

Supplement

* On August 2, there was an M5.5 earthquake at a depth of approximately 10 km in the northern Ibaraki prefecture. The focal mechanism showed a normal fault type with a tension axis in an NE-SW direction. This event occurred within the crust.

* On August 2, there was an M4.6 earthquake at a depth of approximately 50 km under the southern Ibaraki prefecture. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred at the boundary between the Philippine Sea and the continental plates.

* On August 3, there was an M4.6 earthquake at a depth of approximately 45 km under the southern Ibaraki prefecture. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred at the boundary between the Philippine Sea and the continental plates.

* On August 4, there was an M5.3 earthquake at a depth of approximately 65 km under the ocean area near Iriomotejima. The focal mechanism showed a strike-slip fault type with a compression axis in an NE-SW direction (preliminary). This event occurred within the Philippine Sea plate.

• Earthquake in the Northern Ibaraki Prefecture on August 2
• Earthquake under the Southern Ibaraki Prefecture on August 2
• Earthquake under the Southern Ibaraki Prefecture on August 3
• Earthquake near Iriomotejima on August 4

Note: GNSS (Global Navigation Satellite System) is a general name of satellite positioning system such as GPS.

Supplementary Information to the Evaluation of Seismic Activities for July 2017

August 9, 2017
Earthquake Research Committee

1 Major Seismic Activities

The following are the numbers of earthquakes by magnitude (M) that occurred in Japan and the surrounding area during July 2017.

• M4.0 or greater: 78 (74 events in June)
• M5.0 or greater: 78 (74 events in June)
• M6.0 or greater: 1 (0 event in June)

(reference)

• M4.0 or greater: 73 / month (Median number of earthquakes over the past 10 years from 1998 through 2007)
• M5.0 or greater: 9 / month (Median number of earthquakes over the past 35 years from 1973 through 2007)
• M6.0 or greater: 1.4 / month, approximately 17 / year (Average number of earthquakes over the past 84 years from 1924 through 2007)

The following events for the period from July 2016 to late June 2017 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."

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2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There is no supplementary information for Hokkaido region.

to the text

(2) Tohoku Region

There is no supplementary information for Tohoku region.

to the text

(3) Kanto-Chubu Region

–"In the GNSS data for the Tokai region, there has been no change of activity which is likely to be associated with the Tokai earthquake.":
(This evaluation is in agreement with the views presented at the regular meeting on July 24 of the Assessment Committee for Areas under Intensified Measures against Earthquake Disaster (reference below).)

(Reference)

Recent seismic and crustal activity in the Tokai region and its vicinity (Seismological and Volcanological Department, Japan Meteorological Agency, July 24, 2017)
"No change of activity which is likely to be associated with the Tokai earthquake has been observed until now. (Excerpted)"

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(4) Kinki-Chugoku-Shikoku Region

There is no supplementary information for Kinki-Chugoku-Shikoku region.

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(5) Kyushu-Okinawa Region

–"On July 2, there was an M4.5 earthquake at a depth of approximately 10 km in the Aso region, Kumamoto prefecture. (Excerpted.) ":    
In July, 1 earthquake (0 in June) with maximum seismic intensity 4 or over and 1 earthquake (1 in June) with maximum seismic intensity 3 occurred in the Kumamoto and Aso regions, Kumamoto prefecture. A series of seismic activity of the 2016 Kumamoto Earthquakes in the Kumamoto and Aso regions still continues, decaying as a whole, and the seismic activity at present will continue for a while. According to the GNSS observation, postseismic deformation in the area centered on the series of the seismic activity is subsiding as a whole.

–"On July 11, there was an M5.3 earthquake at a depth of approximately 10 km in the Kagoshima Bay. (Excerpted.) ":    
In the vicinity of the event, seismic activity became rather high about December, 2016. Also after the recent event, the activity became high and 28 earthquakes with maximum seismic intensity 1 or over occurred until August 7.

• "The 2016 Kumamoto Earthquakes" (1)
• "The 2016 Kumamoto Earthquakes" (2)
• Postseismic Deformation of the 2016 Kumamoto Earthquake (April 16, M7.3) (1)
• Postseismic Deformation of the 2016 Kumamoto Earthquake (April 16, M7.3) (2)
• Earthquake in the Kagoshima Bay on July 11
• Seismic Activity in the Kagoshima Bay beginning in December 2016
  Hypocenter Re-determination by the DD Method (without the Correlation of Waveforms)

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(6) Other Regions

There is no supplementary information for other regions.

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